Consistent Mastery Grading

I know that the point of a game is to be difficult, if you got everything easily then it wouldn't be fun or challenging. But, I have been playing LoL since BETA, mostly with the same account, and ever since picking it back up (regularly) a year ago, I have been trying to get better mastery levels with my mains. I got my first S- with Teemo a few weeks ago, and it was one of the greatest moments in my gaming career, ever since I have been trying to get that second 'S' rating to get my mastery level 6. Now, again I understand that this shouldn't be an easy feat, and really shows a summoners skill level with certain champs, but I have been queueing up with randoms for almost 3 weeks trying to get this other 'S', every time we either do really great and they surrender, or the bottom lane feeds and we get steamrolled. I just went 8/0/1 as Teemo, queued with random people from across the country, and only got an A+? Not only did I defeat 8 champions, with the help of an awesome team, but I took down two towers, and got the Rift Herald. Of course they surrender early, but I 100% expected to be awarded an S (at least a minus) for this gameplay; I worked very hard and have grown considerably in skill, all while queueing with randoms, and changing builds due to runes/glyphs being removed. What exactly does one have to do in order to consistently get the best grade?